In electric actuating drives for seat adjusters, gear stages are known which are designed as a rolling eccentric stage and which are used as intermediate gear stages or drive input elements for generating a rotating eccentricity for toothed gear stages. A known gear stage of the type which serves as a drive input of a second gear stage with a rotating eccentricity is illustrated in FIG. 14.
Although the expected properties of such rolling eccentric stages, specifically a transmission ratio in the range from 1.5 to 7 with a high efficiency and a low noise level, have indeed been realized in practice, the concept nevertheless has disadvantages which can be compensated only with a comparatively high level of expenditure. The eccentricity required for precise and uniform rolling of the toothed pinion in the ring gear, the magnitude of which eccentricity must remain as precisely constant as possible, arises in the known solutions from the combination of different geometries—for example the drive balls, which push the pinion upward in FIG. 14, a maximum limitation of the movement by the toothing, by the drive output bolts in the pinion bores or by a thrust bearing between the drive input and drive output, and a minimum limitation for example by a support ball. Overall, the relatively large number of components which form the eccentricity result in a system which, as a result of tolerances, load-dependent deformations and internal stresses, is extremely sensitive and susceptible to failure and which, under mass production conditions, can presumably be brought up to a good quality level only with a high level of expenditure.
As a further basic disadvantage, it should be stated that a rolling eccentric of the type can be formed effectively and simply as a drive input element which operates in one plane and with a single pinion (as illustrated in FIG. 14), but the radial bearing forces, proportional to the overall drive output torque, must be absorbed directly and entirely by the rotor bearing rotating at high rotational speed, and there, with increasing loading, lead to increasing power losses and therefore falling efficiency under relatively high operating load. In contrast, if two or ideally three pinions are arranged in planes one above the other in a known way, then the radial forces can support one another—but with the illustrated design, this is not possible by means of a simple arrangement of identical stages one above the other, because even minimal geometric differences of the components involved lead to fundamentally different transmission ratios, and therefore, during extended periods of operation, to a phase offset of the gear stages to one another and/or to stresses in the case of forced synchronicity, and therefore to losses.